How can I tell if my fuel pump is going bad?

We first noticed a strange sound sort of like someone knocking inside an empty 55 gallon drum. The sound was spaced out several seconds apart. It comes from the fuel tank. In order to hear it better, I removed the gas cap. I could hear suction being released and the strange sound stopped. I'm tempted to try a new gas cap before tearing into the tank. Any ideas?

I'm losing antifreeze. Traveled 30 miles to work. 25 miles into the ride the thermastat rose nearly to the H. It was about a minute after I turned on the heat and then traffic came to a stop on PA t...